Update CONTRIBUTING.md
Update CONTRIBUTING.md

Clarify building of single features
rusoto/rusoto,DualSpark/rusoto,mthjones/rusoto,mattico/rusoto,ford-prefect/rusoto,DualSpark/rusoto,DualSpark/rust-aws,DualSpark/rust-aws,mattico/rusoto,rusoto/rusoto,DualSpark/rusoto,SecurityInsanity/rusoto,SecurityInsanity/rusoto,DualSpark/rust-aws,rusoto/rusoto,mattico/rusoto,ford-prefect/rusoto,mthjones/rusoto